With More in Content UI + you can view interface in tabs (or sidebar) rather than floating window. It means that you can change default behavior of commands in Firefox and add-ons (menu items, toolbar button, etc.) from opening an interface in window to opening it in sidebar or tab.
Here are the supported windows:
- Firefox Library (Bookmarks, History and Downloads). All ways to open Library are supported (including Downloads Panel and Downloads Complete notification)
- View Page Source including viewing source of frames
- Firefox Options (it uses built-in Firefox feature that is hidden in prefs)
- Page Info
- DOM Inspector
Want to learn more about this add-on? You can check it out on the Firefox Add-ons website.